1. Field of Invention
This invention relates to steam turbines in general and more particularly to downwardly discharging exhaust hoods for such turbines and more specifically to effecting a decrease of pressure and energy loss in the top portion of such exhaust hoods.
2. Discussion of Prior Art
The steam leaving the last row of blades of steam turbines used in power generation generally flows through an annular passage between the turbine enclosure or casing and the bearing cone into a collector called an xe2x80x9cexhaust hoodxe2x80x9d, from which it discharges into a condenser. The most prevalent type of the exhaust hood is one of xe2x80x9cdownward-dischargingxe2x80x9d design in which the condenser is located below the exhaust hood. This arrangement saves floor space in a power station, but has disadvantages so far as efficient flow in the exhaust hood itself is concerned.
At the top of a downward-discharging exhaust hood, steam leaving the last row of blades of a turbine and after passing through the usual annular flow diffuser follows a tortuous path on its way to the condenser. It is forced by the end and outer walls of the exhaust hood to turn back, that is, to turn in a direction which is essentially opposite to that in which it leaves the turbine. Subsequently, it is further turned downwardly and directed around the turbine casing so that it exits the exhaust hood in a predominantly downward direction as it enters the condenser. A considerable amount of the steam passing through the upper portion of a downwardly discharging exhaust hood therefore changes direction from the time it leaves the annular flow diffuser to the time it reaches the condenser by as much as 270 degrees or more. In contrast, in the bottom portion of the exhaust hood steam exhausting from the annular flow diffuser changes its flow direction only from mainly horizontal to vertically downward or essentially 90 degrees. Consequently, while at the top of the exhaust hood most of the flowing steam changes its direction by approximately 270 degrees before it becomes oriented toward the condenser, followed by additional changes in flow direction caused by the necessity to flow around the turbine casing before it actually enters the condenser, its direction is changed by only about 90 degrees in the bottom of the exhaust hood before entering the condenser. Every turn of a stream of steam, or change of its flow direction, entails a change of its linear momentum, which requires forces to be exerted by the containing walls on the flowing steam causing an increase of steam pressure. As a result, the pressure in the top or upper portion of a downward discharging exhaust hood is appreciably higher than in the bottom portion. This pressure rise in the top portion of the exhaust hood increases the back pressure of the steam turbine and thus decreases the energy available to the turbine to generate power and consequently lowers such turbine""s overall efficiency.
It should also be noted that the turn, or change of direction, of the steam between the end wall and the outer wall of an exhaust hood takes place mainly in the corner region which exists between these two walls. In that corner region the flow separates from the end wall and forms what is known as a xe2x80x9cseparated-flow regionxe2x80x9d in which significant kinetic energy loss occurs as a result of friction. A tightly spiraling separated flow, or vortex, tends to form, which vortex extends in the outer corner region from the top and on both sides of the exhaust hood toward the condenser flange in the general shape of a horseshoe. In addition, the steam flowing initially along the bearing cone separates in the vicinity of the corner which usually exists between the bearing cone outside surface and the exhaust hood end wall forming another xe2x80x9cseparated-flow regionxe2x80x9d filled by another vortex of steam. Energy loss occurs in both of these xe2x80x9cseparated-flow regionsxe2x80x9d as a result of friction of the vortex steam with the walls of the exhaust hood as well as with the passing steam.
Attempts have been made to eliminate the xe2x80x9cseparated-flow regionsxe2x80x9d in the corners of exhaust hoods by placing curved walls or plates over such corners to more smoothly direct the steam flow past them and thus to improve turbine efficiency.
The flow by-pass system of this invention not only eliminates the xe2x80x9cseparated-flow regionsxe2x80x9d and associated energy losses, but, by allowing a significant fraction of the steam which enters the top quadrant of the exhaust hood to by-pass the top and back portions of the exhaust hood there is a resultant decrease of the pressure rise there as a result of decreasing the amount of the turning flow stream as well as its velocity, further significantly improving turbine efficiency. As was stated earlier, lowering of pressure at the exit of a turbine results in an increase of the energy available to the turbine. Lowering of the flow velocity results in lowering of friction losses which are proportional to the square of the flow velocity. There has been a need, therefore, not only for elimination of the xe2x80x9cseparated-flow regionsxe2x80x9d in the corners adjacent to the exhaust hood end wall but also a need to lower the steam pressure and the flow velocity in the upper or top portion of the exhaust hood.
Attempts have been made in the past to decrease energy losses in steam turbine exhaust hoods. For example, U.S. Pat. No. 1,269,998 issued Jun. 18, 1918 to K. Baumann, assigned to Westinghouse, U.S.A., entitled xe2x80x9cSteam Turbinexe2x80x9d discloses a steam turbine having a downwardly curved exhaust at the end of the turbine. In order to better control the flow of steam to the condenser and avoid backup of steam due to vortices and the like, the steam flow is divided up into at least upper and lower streams by partitions or baffle plates. This avoids, it is said, the steam from various portions of the turbine and particularly the top portion and bottom portion from meeting each other at different angles, or from different directions, causing eddies and the like which would interfere with rapid exhaust of steam from the turbine. The uniformity of the travel passage of the steam from the turbine when it enters the condenser or exhaust is thus enhanced. This disclosure does not show an exhaust hood installation directly over a condenser, but is an early example of the widespread continuing practice of using guide vanes to aid in directing turbine exhaust flow.
U.S. Pat. No. 3,791,759 issued Feb. 12, 1974 to J. A. Tetrault, assignor to the U.S. Government, entitled xe2x80x9cTurbine Pressure Attenuation Plenum Chambersxe2x80x9d discloses in a gas turbine the use of pressure attenuation chambers adjacent the exit from the turbine blades which are followed by stationary vanes. Excess gas pressure causes leakage of flow through orifices into such chambers when pressure rises excessively. The reference broadly illustrates the temporary withdrawal of gas from the exhaust to equalize pressure with the intent of trying to reduce circumferential pressure distortion in a turbine which is not provided with a downwardly discharging exhaust hood.
U.S. Pat. No. 3,149,470 issued Sep. 22, 1964 to J. Herzog, assignor to General Electric Company entitled xe2x80x9cLow Pressure Turbine Exhaust Hoodxe2x80x9d discloses a hollow, substantially frusto-conical, flow dividing member disposed inside an exhaust hood co-axial with the turbine rotor which divides the flow from the turbine casing outlet into radially inner and radially outer annular portions which are further sub-divided by additional substantially radial flow guiding walls which form a number of parallel passages leading toward the exhaust hood outlet. One of the two flow annuli is formed between the circular opening of the flow-dividing member and the outer flow guide extending from turbine casing. The other annulus is formed between the circular opening of the flow dividing member and the bearing cone. This prior invention has little if any relation to the present invention in which only a small fraction of the total turbine exhaust flow is by-passed from the top quadrant of the exhaust hood to the bottom portion. There is no physical similarity between these two inventions. In the present invention the space within the exhaust hood remains essentially unchanged by the introduction of the flow by-pass system of the invention. In the Herzog invention the turbine exhaust flow annulus is sub-divided, in proximity of the turbine last stage blades, into two portions.
A variant of U.S. Pat. No. 3,149,470 by J. Herzog is illustrated in FIG. 4-42 on page 106 of K. Cotton""s book: Evaluating and Improving Steam Turbine Performancexe2x80x9d published by Cotton Fact Inc., LSBN#: 0-963339955-0-2, Library of Congress Number 93-0910089 in 1993. It retains the flow-dividing member of the original Herzog design but only in the bottom portion of the exhaust hood. In this variant the exhaust flow sub-division takes place wholly in the bottom portion of the exhaust hood which location is opposite to the location of the inlet vents of the flow by-pass system of the present invention which are in the top portion of the exhaust hood only.
U.S. Pat. No. 4,013,378 issued on Mar. 22, 1977 to J. Herzog assignor to General Electric Company entitled xe2x80x9cAxial Flow Turbine Exhaust Hoodxe2x80x9d discloses the use of a plurality of curved guide vanes of varying curvatures spaced about a circumferential guide ring in the exhaust from a turbine to direct exhaust steam from a generally axial to radial direction, and secondary guide vanes directing steam flow toward the discharge opening of the exhaust hood toward the condenser. There is no by-passing or draining off of some of the steam from the upper or top portion of the exhaust hood and directing it to the lower or bottom portion to decrease the pressure and the flow velocity in the top portion of the exhaust hood.
U.S. Pat. No. 4,214,452 issued on Jul. 29,1980 to G. Riollet et al., assignors to Alsthom-Atlantique of Paris, France, entitled xe2x80x9cExhaust Device for a Condensable-Fluid Axial-Flow Turbinexe2x80x9d discloses in an annular diffuser an extraction-type suction slot or slots through which a fraction of flowing fluid is removed at the outer flow guide so as to make the pressure gradient there either negative or zero. The removed fluid is directed to a lower portion of a two-zone condenser and the remainder to a higher-pressure portion of the two-zone condenser. The location of the removal of fluid at the outer flow guide of the annular exit flow diffuser in the vicinity of the turbine last stage blades distinguishes this invention from the present invention in which fluid is removed in the exhaust hood proper. In addition, the object of this invention is to prevent flow separation from the outer flow guide or xe2x80x9cbreak down of the fluid flowxe2x80x9d there and not to decrease the pressure and flow velocity in the top portion of the exhaust hood which is object of the present invention. Also, since in a Low-pressure (LP) condensing turbine the pressure at the location of the suction slot on the convex side of the diffuser is usually as low or even lower than the pressure in the condenser itself, the Riollet et al. invention can only be used in either a High-pressure (HP) section of the turbine or in the Intermediate-pressure (IP) section of a condensing turbine, or only with a specially-designed condenser having multiple zones.
U.S. Pat. No. 4,326,832 issued to T. Ikeda et al. On Apr. 27, 1982, entitled xe2x80x9cExhaust Outer Casingxe2x80x9d, assigned to Tokyo Shibaura Denki Kabushi Ki Kaisha of Kawasaki, Japan, discloses a side discharge steam exhaust including the use of a guide plate or vane that separates the top discharge steam from lower discharge steam in the exhaust hood and uses curved guide vanes in the hood. However, there is no siphoning off of a portion of the steam exhaust from the upper portion of the exhaust hood and transferring it to the lower portion of the hood.
U.S. Pat. No. 5,174,120 issued to G. J. Silvestri, Jr., assignor to Westinghouse, on Dec. 29, 1992, entitled xe2x80x9cTurbine Exhaust Arrangement for Improved Efficiencyxe2x80x9d discloses a turbine arrangement in which a condenser on the bottom of such arrangement is divided by a plate in the center of the condenser which separates the inlet sections of the tubes of the condenser from the outlet sections of the tubes of the condenser creating a low pressure chamber and a higher pressure chamber, or section, forming thereby a zoned condenser. At the same time the exhaust hood above has partition plates dividing the steam exiting the turbine into left and right half portions. An increased efficiency is claimed. Although in the Silvestri arrangement the turbine exhaust is divided into separate zones, in the arrangement of the present invention siphoning off of elevated pressure steam from the top of an exhaust hood and transferring it to the bottom of the exhaust hood is not shown.
U.S. Pat. No. 5,257,906 issued on Nov. 30, 1993 to L. Gray et al., assignors to Westinghouse, entitled xe2x80x9cExhaust System for a Turbomachinexe2x80x9d discloses a steam turbine arrangement in which the outer edge of the outer flow guide approaches a vertical orientation and is overall more extended on the bottom and in which an upper baffle within the exhaust hood dissipates the horseshoe-shaped vortex in the upper section and sides of the exhaust hood by xe2x80x9ccrowdingxe2x80x9d the vortex against such baffle, thus apparently minimizing formation of such vortex and preventing it from expanding and growing. There is no disclosure of a means for siphoning of fluid from the top portion of the exhaust hood to the bottom portion.
Russian Patent 1,724,903 apparently issued on Apr. 7, 1992, shows a steam turbine arrangement in which a moisture removal slot is provided between the casing and the outer flow guide just upstream of the turbine last stage blade. While it is possible that some steam would also enter such slot or passage, any such steam would be bled off before the last blades into a specially-provided annular passage extending all around the circumference of the turbine well before any subsequent exhaust hood and would not contribute to any significant extent to equalization of pressure in the top and bottom portions of the exhaust hood, which pressure difference is caused by the tortuous path which steam must follow in the top portion of the exhaust hood.
While the problems detrimental to performance of steam turbines associated with the build-up of pressure as well as with the horseshoe-shaped vortex formation in the top portion of a downwardly discharging exhaust hood have been known, and there has been a need to solve such problems, see, for example, the discussion in the background of U.S. Pat. No. 5,257,906 to Gray et al., so far as the present inventor is aware no one has proposed bleeding off a fraction of the steam flow from the top portion and feeding it back into the bottom portion or to the condenser itself nor using a flow by-pass system located over the corners of the hood which tend to produce xe2x80x9cseparated-flow regionsxe2x80x9d so as to decrease the pressure differential between the top and bottom portions of a downwardly discharging exhaust hood and to decrease the flow velocity in the top portion upstream of the tortuous path leading to the condenser inlet.
It is an object of this invention, therefore, to reduce the pressure differential between the top and bottom portions of a downward-discharging exhaust hood of a steam turbine installation by bleeding off, or aspirating, some of the steam from the top portion into the bottom portion or directly to the condenser through a flow by-pass.
It is a further object of the invention to provide a flow by-pass system at the end of a downward-discharging exhaust hood for transferring steam from the top portion of the exhaust hood to the bottom portion in order to decrease the pressure build-up and the flow velocity in the top portion.
It is a still further object of the invention to provide a flow by-pass between the top and bottom portions of a downward-discharging exhaust hood wherein the by-pass has inlets and outlets in the vicinity of the corners located at the end walls of the exhaust hood where normally flow separation occurs, thus not only reducing the pressure differential between the top and bottom portions, but also eliminating such separated flow regions in which energy loss occurs.
It is a still further object of the invention to provide a by-pass for steam from the top portion of a downward-discharging exhaust hood in a steam turbine installation to the condenser region below in which the outlet or outlets are so positioned that steam is aspirated from the top portion to the lower condenser region effectively tending toward equalization of the steam pressure between the top and bottom portions of the exhaust hood.
It is a still further object of this invention to decrease the average pressure drop across a downward-discharging exhaust hood of a steam turbine by decreasing the amount by which the pressure in the top portion of such exhaust hood is raised as a result of turning of steam on its way to the condenser and to reduce energy loss caused by skin friction there and to reduce the separated flow region or regions, in which additional energy loss occurs as a result of friction, thus increasing the amount of work produced by the turbine, that is, increasing its efficiency.
Other objects and advantages of the invention will become evident from the following description and explanation in conjunction with the appended drawings.
In downward-discharging exhaust hoods of condensing steam turbine installations the exhaust steam entering the top portion of the hood is turned upwardly and then backward and around the sides of the turbine casing toward the condenser positioned under the hood, while the exhaust steam entering the bottom portion of the hood is immediately turned downwardly toward the condenser. The extra 180 degrees of change of flow direction for the steam in the top portion of the hood, which is in fact even larger because of the necessity to flow around the turbine casing, represents in effect a restriction to flow there resulting in a higher pressure in the top portion of the hood than in the bottom portion. This higher pressure is transferred back through the usual diffuser as higher back pressure acting against the steam exiting the turbine resulting in a decrease of the energy available to the turbine to do work and thus in power loss. The higher pressure in the top portion of the exhaust hood is decreased and brought closer to the lower pressure prevailing in the bottom portion of the hood in accordance with this invention by providing a flow by-pass system, or arrangement, between the top portion and either the bottom portion of the exhaust hood or the condenser through which some of the steam from the top portion of the exhaust hood is transported more directly to the lower pressure zone below. The take off locations for steam, or the inlets for steam to the by-pass passages, are preferably located near the upstream edges of the normal xe2x80x9cseparated flow regionsxe2x80x9d near the corners of the exhaust hood effectively eliminating such undesirable flow regions. The by-pass conduits, forming internal by-pass passages, are preferably formed by the provision of walls, or plates, within the interior of the end of the exhaust hood, with appropriate inlet and exit openings. The inlet openings are positioned in the higher-pressure zone facing the flowing steam so that steam is pushed or xe2x80x9crammedxe2x80x9d into said inlets. The exit openings are located in the lower pressure zone so that steam tends to be aspirated from the flow by-pass passages into such zone. Alternative constructions are possible.